Lubricants for industrial and automotive uses are, usually, single phase, constant composition materials made of basestocks to which various performance enhancing additives may be incorporated. Typically, a lubricant is selected to optimize the performance, function and protection of lubricated systems such as gears, cam-follower pairs, roller bearings, hydrodynamic bearings or pumps.
To optimize system performance, the lubricant is formulated by selecting one or more basestocks and additives which will meet system needs when combined. Viscosity properties are an important consideration when formulating a lubricant because the appropriate viscosity balances energy loss because of viscous drag with wear because of diminished oil film thickness. Antiwear and antiscuff additives can help protect surfaces when the oil film between them thins. Basestocks, depending on their composition, can have various beneficial properties such as antioxidancy, good viscosity index and low traction.
While it is possible to optimize lubricant selection for a single phase lubricant, optimum performance under the prevailing operating conditions (such as speed, load and temperature) can be compromised when the conditions change or when the same lubricant is used to lubricate several parts, each part having its own unique lubricant needs. Frequently, one lubricant is used to lubricate a wide variety of machines, for the sake of simplicity.
The use of a single phase lubricant requires a compromise between premature machine failure due to wear, fatigue or scuffing, because the lubricant is of insufficient viscosity under certain operating conditions, with excessive energy loss or overheating, because the lubricant viscosity is too high for other operating conditions. Since machine or plant operators prefer to minimize downtime, the compromise usually favors high viscosity lubricants which reduce equipment failure but cause excess energy loss. Additionally, where temperatures vary widely, the viscosity selected may be for the weakest link component at its highest operating temperature but because of the relationship between viscosity and temperature, much energy is wasted since the lubricant viscosity is too high under the normally lower operating temperatures.
A two-phase lubricant for improving the low temperature starting of an automobile engine is disclosed in French Patent No. 2,205,931. There, a process is disclosed in which a more dense lower viscosity phase and a less dense higher viscosity phase are combined such that a homogeneous phase, with lubricating properties characteristic of conventional engine lubricants, exists when the engine is at its operating temperature. When the engine is cool, the phases separate so that on starting the engine under low temperature conditions only the low viscosity more dense fluid is drawn into the oil pump. This improves engine cranking speeds because of reduced viscous drag and provides easier starting. The disclosed lubricants are hydrocarbons and esters, the heavier ester phase has the lower viscosity. The objective of the patent is to ease cold starting of an automobile engine. The patent describes the disadvantages associated with polymers as viscosity index enhancers because they degrade with use and cause the oil to lose viscosity. Thus, the patent suggests the use of the two-component system as an alternative to polymers because it provides good low temperature starting without viscosity degradation over time. The lubricant phases mix as the engine warms-up and the mixed lubricant behaves in the conventional manner, demonstrating large viscosity variations with temperature.
The moving parts of industrial machines are often actuated by liquid, usually oil, sometimes water-based emulsions, that is under pressure. The system used to apply the liquid is a hydraulic system, usually a contained system, which includes a reservoir, a motor driven pump, control valves, a fluid motor and piping which connects these components. Lubricating oil is often selected because it is useful for pressure transmission and controlled wear and it minimizes friction and wear of moving parts and inhibits corrosion. Hydraulic fluids with low pour points and high viscosity indexes (VI) are formulated for use in mobile and marine applications that are subject to temperature extremes during service. With these high VI fluids, the goal is to obtain effective and consistent hydraulic actuation under extremes of high and low temperatures. Although there are many high VI hydraulic fluids available which attempt to meet this objective, it is very difficult to do with one fluid phase that has fairly constant properties.